[A-HW23] 同位体水文学2021

[AHW23-P04] The influence of weathering and hydrological cycles on Li isotopic compositions in river water and groundwater of the Ganges–Brahmaputra–Meghna River system in Bangladesh

*吉村 寿紘¹、荒岡 大輔²、川幡 穂高³、Hossain Zakir⁴、大河内 直彦¹ (1.海洋研究開発機構、2.産業技術総合研究所、3.東京大学大気海洋研究所、4.ジョソール科学技術大学)

キーワード：風化、バングラデシュ、リチウム同位体比、地下水、ガンジス川

The silicate weathering of continental rocks plays a vital role in determining ocean chemistry and global climate. Spatiotemporal variations in Li isotope ratio (δ⁷Li) in terrestrial waters identify regimes of current and past weathering processes. Here we examine (1) monthly dissolved δ⁷Li variation in the Ganges River’s lower reaches, and (2) spatiotemporal variation of river water of the Brahmaputra, Meghna rivers, and groundwater in Bangladesh. From the beginning to maximum flood discharges of the rainy season (i.e., from June to September), Li concentrations and δ⁷Li in the Ganges River show remarkable changes, with a large influence from the Himalayan sources. However, most Li discharge across the rainy season is a steady-state condition and strongly influenced by the secondary mineral formation in the low-altitude floodplain. For groundwater samples shallower than ~60 m depth, both δ⁷Li and Li/Na are highly scattered regardless of the sampling region, suggesting the variable extent of fractionation. For deep groundwater (70–310 m) with a long residence time of about 3,000 to 20,000 years, the lower δ⁷Li values indicate more congruent weathering. These results suggest that Li isotope fractionation in rivers and groundwater depends on the timescale of water-mineral interaction, which plays an essential role in determining the isotopic signature of terrestrial Li inputs to the ocean.